It has long been known that soda-lime-silica glasses may be rendered green in color by the addition to the batch of various amounts of iron. It is somewhat conventional, in this respect, to express the total iron content as a weight percent expressed as Fe.sub.2 O.sub.3 even though the iron may exist in various valence states in the glass. However, it is also well known that the balance within the total iron content of ferrous oxide (FeO) and ferric oxide (Fe.sub.2 O.sub.3) can make a significant difference in the shade of green obtained, as well as in other characteristics of the glass. Generally speaking, for example, as the concentration of FeO increases in relation to Fe.sub.2 O.sub.3, the color of the glass shifts from a yellow-green to a darker green or blue-green of lower visible transmittance.
Other characteristics of the glass change as the relationship of FeO to Fe.sub.2 O.sub.3 (i e. ferrous to ferric content) shifts. For example, it is further known that iron in the ferrous state (FeO) is an absorber of infrared energy in glass. Unfortunately, as the FeO content increases at the expense of Fe.sub.2 O.sub.3, the ultraviolet energy absorption decreases because Fe.sub.2 O.sub.3 is a UV absorber. It has been further reported that at certain relatively high FeO concentrations thought desirable for high infrared absorption, heat penetration into the glass melt is severely restricted and special melting and refining apparatus has to be employed to achieve a homogeneous glass.
Representative of the above knowledge and other relevant information known to those skilled in the art with regard to making iron-containing soda-lime-silica green glasses having relatively high visible, low ultraviolet and low infrared transmittances, prior to this invention, are the discussions and inventions disclosed in U.S. Pat. No. 4,792,536 and U.S. Pat. No. 5,077,133 and the prior art cited and discussed therein.
The glasses in the preferred embodiments in both of these prior art patents seek, in their own way, to balance the ferric and ferrous contents by altering oxidation states to shift their respective amounts in order to achieve certain infrared absorption levels of the inventors' choosing, while maintaining the needed degree of visible transmittance. Then, in order to obtain the necessary degree of ultraviolet absorption, and thus appropriate levels of total solar transmittance, both patents teach to add cerium oxide (CeO.sub.2) to the composition. U.S. Pat. No. 4,792,536 does disclose certain other UV absorbing additives that may be used in place of CeO.sub.2, such as TiO.sub.2, V.sub.2 O.sub.5 and MoO.sub.3. U.S. Pat. No. 5,077,133, on the other hand, states that it is critical to use CeO.sub.2 in the practice of its invention, but contemplates that some of the CeO.sub.2 may be complemented by using TiO.sub.2 in admixture with it.
All of these UV absorbing additives are expensive, particularly CeO.sub.2 . In addition, they create their own problems in the composition. For example, additives other than CeO.sub.2 are reported as having an adverse effect on visible transmittance. On the other hand, in addition to being expensive and having its own adverse effect upon visible transmittance, CeO.sub.2 inhibits reduction of Fe.sub.2 O.sub.3 to FeO (i.e. ferric to ferrous iron).
Another problem regarding the manufacture of acceptable green glasses, as reported in U.S. Pat. No. 4,792,536, is that the presence of sulfur in the glass inhibits reduction of Fe.sub.2 O.sub.3 to FeO. Unfortunately sulfur, for example in the form of sodium sulfate ("Salt Cake" Na.sub.2 SO.sub.4) is a known inexpensive, and valuable refining agent for soda-lime-silica glasses, particularly those to be made by the conventional "float" process as flat glass for architectural and/or automotive purposes. Because of its inhibiting effect on the reduction of Fe.sub.2 O.sub.3 to FeO, U.S. Pat. No. 4,792,536 seeks to severely limit the amount of sulfur (SO.sub.3) in the glass composition. Then, to achieve its overall purposes, this patent minimizes the total iron content, preferably to about 0.45% to 0.65% by weight of the total glass composition (to preserve visible transmittance) and forms the glass under reducing conditions, so as to create from this modest amount of iron greater than 35% and most preferably at least 50% by weight of the total iron content in the ferrous state (as FeO). Infrared transmittances no greater than 15% are said to be achieved.
According to the examples presented in the '536 patent, when the glasses are reduced to the desired level to obtain this IR transmittance, and because of the low total iron content, UV transmittance increases due to the small amount of ferric iron remaining. This, in turn, necessitates additives such as CeO.sub.2, etc. for UV absorption. Thusly, in most of the examples visible transmittance (reported as "LT.sub.A ") still rarely comes close to reaching 70%, a desirable, and sometimes necessary, level of visible transmittance for automotive glasses. For example, of the glass compositions of the patented invention reported in the Tables in this patent which do not employ special UV absorbing additives (e.g. CeO.sub.2), only Composition 14 has a visible transmittance of 70% or greater. (Example 11 does, but it is said to be insufficiently reduced, and employs 1.0% cerium oxide). Because Composition 14 does not employ CeO.sub.2 or any other of the above-referenced UV absorbing oxides (i.e. TiO.sub.2, MoO.sub.3 or V.sub.2 O.sub.5) ultraviolet transmittance, even at 5 mm, is a very high 51.3%. This latter value renders these glasses rather unacceptable for certain automotive uses. In some instances, furthermore, the "tilt" toward a bluish-green color also makes many of these compositions undesirable for certain customer specifications.
This '536 patent also discloses four "prior art" glass compositions. Composition 1 is a conventional green glass whose solar blocking is shown to be inferior. Compositions 2-4, while somewhat closer in iron content to the glasses of our invention, nevertheless suffer in one or more respects in their solar management characteristics, even when reduced in thickness to less than 5 mm, as compared to the glasses of our invention. In this respect, Composition 2 is actually a different glass than our invention because of its high Al.sub.2 O.sub.3 and K.sub.2 O content, making it difficult to produce by the "float process", a process particularly preferred for use in our invention. Composition 3 is also significantly different in its SO.sub.3 content from that contained in the glasses of our invention (a rather narrow and critical range of about 0.20-0.25% by weight, as stated below), making the glass of Composition 3 difficult to refine. Its FeO/Fe.sub.2 O.sub.3 content is also quite high compared to our invention, thus resulting in a relatively high UV transmittance when its thickness is less than 5 mm. Composition 4 is so highly reduced that it has a very high UV transmittance compared to our glasses. In addition, this composition, producible only by special techniques is not applicable for use in the float process.
In contrast to the iron minimizing, low sulfur compositions of U.S. Pat. No. 4,792,536, the glass compositions of U.S. Pat. No. 5,077,133 seek to achieve green glasses through the use of relatively high total iron content concentrations. These glasses also employ conventional salt cake in their batch for refining purposes and are said to require only conventional melting and refining apparatus. They are generally colored toward the yellow-green side, as opposed to the bluer greens of the '536 patent.
U.S. Pat. No. 5,077,133 seeks to differentiate itself from the above-referenced U.S. Pat. No. 4,792,536 (at col. 2, line 40 et seq.). It does so, in part, by pointing out the rather low levels of iron employed in this latter patent and the resulting characteristics achieved thereby in the final glass. U.S. Pat. No. 5,027,133 states that, in contrast, its glasses employ higher levels of total iron content, and, by carefully controlling the ratio of FeO to total iron content (expressed as Fe.sub.2 O.sub.3) and the critical use of certain specific concentrations of CeO.sub.2 (alone or with TiO.sub.2) superior color and transmittance characteristics are achieved.
More specifically, U.S. Pat. No. 5,077,133 states that, using high total iron content within certain limits, certain specific FeO to total iron content ratios (by reduction) and specific values of CeO.sub.2, Illuminant A visible light transmittance is greater than 70%, total solar energy transmittance is less than about 46%, ultraviolet radiation transmittance is less than about 38% and preferably is less than about 34%. This patent further states that Illuminant C dominant wavelength is from about 498 to about 525 nm, and that the glass displays a color purity of about 2% to 4% compared to the blue glass of the prior '356 patent which is said to be up to about 10%. At least some of these characteristics are, of course, thickness dependent and for this reason, it is stated that these combined results are achieved with glasses (single sheet or more) having a total thickness of 3 mm to 5 mm.
While U.S. Pat. No. 5,077,133 states that it achieves "low infrared energy" transmittance (e.g. col. 7, lines 1-2), it does not disclose what that value, or range of values, is. Using the formula presented in U.S. Pat. No. 4,792,536 (at col. 13, line 50), it can be calculated that IR transmittance (TSIR) in the '133 patent is on the order of about 17% to 33%.
The transmittance, purity and color characteristics reported in U.S. Pat. No. 5,077,133 are desirable characteristics to achieve, particularly in the automotive field for windshields, side and rear windows, and other automotive glass. However, the use of CeO.sub.2, either alone or with TiO.sub.2, is an undesirable and detrimental drawback to the invention disclosed in that patent, as it is in the prior U.S. Pat. No. 4,792,536.
It is, therefore, apparent from the above that there exists a need in the art for a glass composition which achieves the transmittance and color properties generally as disclosed in U.S. Pat. No. 5,077,133, without having to employ special UV absorbing additives such as CeO.sub.2, heretofore believed necessary in the prior art to achieve proper UV absorption.
It is a purpose of this invention to fulfill this and other needs in the art which will become more apparent to the skilled artisan once given the following disclosure.